price check

i'm a 16 year old kid that has been mowing 10 lawns the entire spring and summer. i'm sick of my 21" mower. my neighbor up the street has a 36" TORO T-Bar Fixed Deck ProLine Gear Mid-Size mower. It has a 15 horsepower Kohler motor. it is about 2 1/2 years old. what sounds like a good price for him to sell it? what would a professional service pay him for it? what should i pay him for it??